The use of polymers for pipes for various purposes, such as fluid transport, e.g. transport of liquids or gases such as water or natural gas is known. It is common for the fluid to be pressurised in these pipes. Such pipes can be made of polyethylene such as medium density polyethylene (MDPE) or high density polyethylene (HDPE), typically having density of about 950 kg/m3.
Pipes can be manufactured using various techniques such as RAM extrusion or screw extrusion. Screw extrusion is one of the core operations in polymer processing and is also a key component in many other processing operations. An important aim in a screw extrusion process is to build pressure in a polymer melt so that it can be extruded through a die.
Crosslinking improves parameters such as heat deformation resistance and therefore pipes for hot water applications, such as pipes for floor heating, or for hot water distribution, are usually made of crosslinked polyethylene (PEX).
In order to improve the crosslinking response and hence reduce the consumption of crosslinking agent, e.g. peroxide, when crosslinking pipes of polyethylene, it is generally desired to use an ethylene polymer of relatively low melt flow rate (MFR), i.e. high molecular weight. However, this results in the drawback of poor processability, i.e. a reduced line speed at extrusion.
Another problem that may arise is insufficient melt strength when using polymers having a higher MFR in order to achieve better extrudability. In a pipe manufacturing system, where pipes are extruded and crosslinked in a separate system, melt strength is required to keep dimensional stability of the partly molten, uncrosslinked pipe until it becomes crosslinked. In the worst case, lack of melt strength means that it may not be possible to prepare a pipe from the polymer, because the polymer parison collapses when it exits from the extruder. Higher MFR resins also have poorer crosslinkability, which means that a larger amount of crosslinking agent or a stronger irradiation dose must be used.